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[PATCH v9] Avoid orphaned objects dependencies 425+ messages / 2 participants [nested] [flat]
* [PATCH v9] Avoid orphaned objects dependencies @ 2024-03-29 15:43 Bertrand Drouvot <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Bertrand Drouvot @ 2024-03-29 15:43 UTC (permalink / raw) It's currently possible to create orphaned objects dependencies, for example: Scenario 1: session 1: begin; drop schema schem; session 2: create a function in the schema schem session 1: commit; With the above, the function created in session 2 would be linked to a non existing schema. Scenario 2: session 1: begin; create a function in the schema schem session 2: drop schema schem; session 1: commit; With the above, the function created in session 1 would be linked to a non existing schema. To avoid those scenarios, a new lock (that conflicts with a lock taken by DROP) has been put in place when the dependencies are being recorded. With this in place, the drop schema in scenario 2 would be locked. Also, after the new lock attempt, the patch checks that the object still exists: with this in place session 2 in scenario 1 would be locked and would report an error once session 1 committs (that would not be the case should session 1 abort the transaction). If the object is dropped before the new lock attempt is triggered then the patch would also report an error (but with less details). The patch adds a few tests for some dependency cases (that would currently produce orphaned objects): - schema and function (as the above scenarios) - alter a dependency (function and schema) - function and arg type - function and return type - function and function - domain and domain - table and type - server and foreign data wrapper --- src/backend/catalog/aclchk.c | 1 + src/backend/catalog/dependency.c | 83 +++++++++-- src/backend/catalog/heap.c | 7 +- src/backend/catalog/index.c | 16 ++- src/backend/catalog/objectaddress.c | 57 ++++++++ src/backend/catalog/pg_aggregate.c | 2 +- src/backend/catalog/pg_attrdef.c | 1 + src/backend/catalog/pg_cast.c | 2 +- src/backend/catalog/pg_collation.c | 1 + src/backend/catalog/pg_constraint.c | 10 +- src/backend/catalog/pg_conversion.c | 2 + src/backend/catalog/pg_depend.c | 34 ++++- src/backend/catalog/pg_operator.c | 2 +- src/backend/catalog/pg_proc.c | 12 +- src/backend/catalog/pg_publication.c | 5 + src/backend/catalog/pg_range.c | 3 +- src/backend/catalog/pg_type.c | 18 ++- src/backend/catalog/toasting.c | 1 + src/backend/commands/alter.c | 3 + src/backend/commands/amcmds.c | 1 + src/backend/commands/cluster.c | 2 + src/backend/commands/event_trigger.c | 1 + src/backend/commands/extension.c | 4 +- src/backend/commands/foreigncmds.c | 7 + src/backend/commands/functioncmds.c | 3 +- src/backend/commands/indexcmds.c | 2 + src/backend/commands/opclasscmds.c | 18 +++ src/backend/commands/policy.c | 2 + src/backend/commands/proclang.c | 2 +- src/backend/commands/sequence.c | 1 + src/backend/commands/statscmds.c | 3 + src/backend/commands/tablecmds.c | 10 ++ src/backend/commands/trigger.c | 18 ++- src/backend/commands/tsearchcmds.c | 8 +- src/backend/commands/typecmds.c | 4 + src/backend/rewrite/rewriteDefine.c | 1 + src/backend/utils/errcodes.txt | 1 + src/include/catalog/dependency.h | 9 +- src/include/catalog/objectaddress.h | 1 + .../expected/test_dependencies_locks.out | 129 ++++++++++++++++++ src/test/isolation/isolation_schedule | 1 + .../specs/test_dependencies_locks.spec | 89 ++++++++++++ 42 files changed, 534 insertions(+), 43 deletions(-) 34.8% src/backend/catalog/ 16.2% src/backend/commands/ 28.0% src/test/isolation/expected/ 17.4% src/test/isolation/specs/ 3.3% src/ diff --git a/src/backend/catalog/aclchk.c b/src/backend/catalog/aclchk.c index 143876b77f..6b91402a9f 100644 --- a/src/backend/catalog/aclchk.c +++ b/src/backend/catalog/aclchk.c @@ -1413,6 +1413,7 @@ SetDefaultACL(InternalDefaultACL *iacls) referenced.objectId = iacls->nspid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_AUTO); } } diff --git a/src/backend/catalog/dependency.c b/src/backend/catalog/dependency.c index d4b5b2ade1..42293220fd 100644 --- a/src/backend/catalog/dependency.c +++ b/src/backend/catalog/dependency.c @@ -1517,6 +1517,68 @@ AcquireDeletionLock(const ObjectAddress *object, int flags) } } +/* + * depLockAndCheckObject + * + * Lock the object that we are about to record a dependency on. + * After it's locked, verify that it hasn't been dropped while we + * weren't looking. If the object has been dropped, this function + * does not return! + */ +void +depLockAndCheckObject(const ObjectAddress *object) +{ + char *object_description; + + if (isObjectPinned(object)) + return; + + /* + * Those don't rely on LockDatabaseObject() when being dropped (see + * AcquireDeletionLock()). Also it looks like they can not produce + * orphaned dependent objects when being dropped. + */ + if (object->classId == RelationRelationId || object->classId == AuthMemRelationId) + return; + + object_description = getObjectDescription(object, true); + + /* assume we should lock the whole object not a sub-object */ + LockDatabaseObject(object->classId, object->objectId, 0, AccessShareLock); + + /* check if object still exists */ + if (!ObjectByIdExist(object)) + { + if (object_description) + ereport(ERROR, + (errcode(ERRCODE_DEPENDENT_OBJECTS_DOES_NOT_EXIST), + errmsg("%s does not exist", object_description))); + else + ereport(ERROR, + (errcode(ERRCODE_DEPENDENT_OBJECTS_DOES_NOT_EXIST), + errmsg("a dependent object does not exist"))); + } + + if (object_description) + pfree(object_description); + + return; +} + +void +depLockAndCheckObjects(const ObjectAddress *object, int nobject) +{ + int i; + + if (nobject < 0) + return; + + for (i = 0; i < nobject; i++, object++) + depLockAndCheckObject(object); + + return; +} + /* * ReleaseDeletionLock - release an object deletion lock * @@ -1562,13 +1624,8 @@ recordDependencyOnExpr(const ObjectAddress *depender, /* Scan the expression tree for referenceable objects */ find_expr_references_walker(expr, &context); - /* Remove any duplicates */ - eliminate_duplicate_dependencies(context.addrs); - - /* And record 'em */ - recordMultipleDependencies(depender, - context.addrs->refs, context.addrs->numrefs, - behavior); + /* Record all of them (this includes duplicate elimination) */ + lock_record_object_address_dependencies(depender, context.addrs, behavior); free_object_addresses(context.addrs); } @@ -1652,9 +1709,12 @@ recordDependencyOnSingleRelExpr(const ObjectAddress *depender, /* Record the self-dependencies with the appropriate direction */ if (!reverse_self) + { + depLockAndCheckObjects(self_addrs->refs, self_addrs->numrefs); recordMultipleDependencies(depender, self_addrs->refs, self_addrs->numrefs, self_behavior); + } else { /* Can't use recordMultipleDependencies, so do it the hard way */ @@ -1664,6 +1724,7 @@ recordDependencyOnSingleRelExpr(const ObjectAddress *depender, { ObjectAddress *thisobj = self_addrs->refs + selfref; + depLockAndCheckObject(depender); recordDependencyOn(thisobj, depender, self_behavior); } } @@ -1672,6 +1733,7 @@ recordDependencyOnSingleRelExpr(const ObjectAddress *depender, } /* Record the external dependencies */ + depLockAndCheckObjects(context.addrs->refs, context.addrs->numrefs); recordMultipleDependencies(depender, context.addrs->refs, context.addrs->numrefs, behavior); @@ -2737,11 +2799,12 @@ stack_address_present_add_flags(const ObjectAddress *object, * removing any duplicates. */ void -record_object_address_dependencies(const ObjectAddress *depender, - ObjectAddresses *referenced, - DependencyType behavior) +lock_record_object_address_dependencies(const ObjectAddress *depender, + ObjectAddresses *referenced, + DependencyType behavior) { eliminate_duplicate_dependencies(referenced); + depLockAndCheckObjects(referenced->refs, referenced->numrefs); recordMultipleDependencies(depender, referenced->refs, referenced->numrefs, behavior); diff --git a/src/backend/catalog/heap.c b/src/backend/catalog/heap.c index a122bbffce..113b275fe1 100644 --- a/src/backend/catalog/heap.c +++ b/src/backend/catalog/heap.c @@ -843,6 +843,7 @@ AddNewAttributeTuples(Oid new_rel_oid, ObjectAddressSubSet(myself, RelationRelationId, new_rel_oid, i + 1); ObjectAddressSet(referenced, TypeRelationId, tupdesc->attrs[i].atttypid); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); /* The default collation is pinned, so don't bother recording it */ @@ -851,6 +852,7 @@ AddNewAttributeTuples(Oid new_rel_oid, { ObjectAddressSet(referenced, CollationRelationId, tupdesc->attrs[i].attcollation); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); } } @@ -1471,7 +1473,7 @@ heap_create_with_catalog(const char *relname, add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); } @@ -3393,7 +3395,7 @@ StorePartitionKey(Relation rel, } } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); /* @@ -3409,6 +3411,7 @@ StorePartitionKey(Relation rel, ObjectAddressSubSet(referenced, RelationRelationId, RelationGetRelid(rel), partattrs[i]); + depLockAndCheckObject(&myself); recordDependencyOn(&referenced, &myself, DEPENDENCY_INTERNAL); } diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c index 55fdde4b24..26201e7610 100644 --- a/src/backend/catalog/index.c +++ b/src/backend/catalog/index.c @@ -1144,7 +1144,7 @@ index_create(Relation heapRelation, add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_AUTO); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_AUTO); free_object_addresses(addrs); } @@ -1157,9 +1157,11 @@ index_create(Relation heapRelation, if (OidIsValid(parentIndexRelid)) { ObjectAddressSet(referenced, RelationRelationId, parentIndexRelid); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_PARTITION_PRI); ObjectAddressSet(referenced, RelationRelationId, heapRelationId); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_PARTITION_SEC); } @@ -1185,7 +1187,7 @@ index_create(Relation heapRelation, add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); /* Store dependencies on anything mentioned in index expressions */ @@ -1987,6 +1989,14 @@ index_constraint_create(Relation heapRelation, */ ObjectAddressSet(myself, ConstraintRelationId, conOid); ObjectAddressSet(idxaddr, RelationRelationId, indexRelationId); + + /* + * CommandCounterIncrement here to ensure the new constraint entry is + * visible when we'll check of object existence when recording the + * dependencies. + */ + CommandCounterIncrement(); + depLockAndCheckObject(&myself); recordDependencyOn(&idxaddr, &myself, DEPENDENCY_INTERNAL); /* @@ -1998,9 +2008,11 @@ index_constraint_create(Relation heapRelation, ObjectAddress referenced; ObjectAddressSet(referenced, ConstraintRelationId, parentConstraintId); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_PARTITION_PRI); ObjectAddressSet(referenced, RelationRelationId, RelationGetRelid(heapRelation)); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_PARTITION_SEC); } diff --git a/src/backend/catalog/objectaddress.c b/src/backend/catalog/objectaddress.c index 7b536ac6fd..6d7abd3738 100644 --- a/src/backend/catalog/objectaddress.c +++ b/src/backend/catalog/objectaddress.c @@ -2590,6 +2590,63 @@ get_object_namespace(const ObjectAddress *address) return oid; } +/* + * ObjectByIdExist + * + * Return whether the given object exists. + * + * Works for most catalogs, if no special processing is needed. + */ +bool +ObjectByIdExist(const ObjectAddress *address) +{ + HeapTuple tuple; + int cache; + const ObjectPropertyType *property; + + property = get_object_property_data(address->classId); + + cache = property->oid_catcache_id; + + if (cache >= 0) + { + /* Fetch tuple from syscache. */ + tuple = SearchSysCache1(cache, ObjectIdGetDatum(address->objectId)); + + if (!HeapTupleIsValid(tuple)) + { + return false; + } + + ReleaseSysCache(tuple); + + return true; + } + else + { + Relation rel; + ScanKeyData skey[1]; + SysScanDesc scan; + + rel = table_open(address->classId, AccessShareLock); + + ScanKeyInit(&skey[0], + get_object_attnum_oid(address->classId), + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(address->objectId)); + + scan = systable_beginscan(rel, get_object_oid_index(address->classId), true, + NULL, 1, skey); + + /* we expect exactly one match */ + tuple = systable_getnext(scan); + systable_endscan(scan); + table_close(rel, AccessShareLock); + + return (HeapTupleIsValid(tuple)); + } +} + /* * Return ObjectType for the given object type as given by * getObjectTypeDescription; if no valid ObjectType code exists, but it's a diff --git a/src/backend/catalog/pg_aggregate.c b/src/backend/catalog/pg_aggregate.c index 90fc7db949..9b5c8c8fb0 100644 --- a/src/backend/catalog/pg_aggregate.c +++ b/src/backend/catalog/pg_aggregate.c @@ -805,7 +805,7 @@ AggregateCreate(const char *aggName, add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); return myself; } diff --git a/src/backend/catalog/pg_attrdef.c b/src/backend/catalog/pg_attrdef.c index 003ae70b4d..47b7fe75ad 100644 --- a/src/backend/catalog/pg_attrdef.c +++ b/src/backend/catalog/pg_attrdef.c @@ -178,6 +178,7 @@ StoreAttrDefault(Relation rel, AttrNumber attnum, colobject.objectId = RelationGetRelid(rel); colobject.objectSubId = attnum; + depLockAndCheckObject(&colobject); recordDependencyOn(&defobject, &colobject, attgenerated ? DEPENDENCY_INTERNAL : DEPENDENCY_AUTO); diff --git a/src/backend/catalog/pg_cast.c b/src/backend/catalog/pg_cast.c index 5a5b855d51..2593bb3634 100644 --- a/src/backend/catalog/pg_cast.c +++ b/src/backend/catalog/pg_cast.c @@ -121,7 +121,7 @@ CastCreate(Oid sourcetypeid, Oid targettypeid, add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, behavior); + lock_record_object_address_dependencies(&myself, addrs, behavior); free_object_addresses(addrs); /* dependency on extension */ diff --git a/src/backend/catalog/pg_collation.c b/src/backend/catalog/pg_collation.c index 7f2f701229..43ceb426d4 100644 --- a/src/backend/catalog/pg_collation.c +++ b/src/backend/catalog/pg_collation.c @@ -218,6 +218,7 @@ CollationCreate(const char *collname, Oid collnamespace, referenced.classId = NamespaceRelationId; referenced.objectId = collnamespace; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); /* create dependency on owner */ diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index 3baf9231ed..91af823ee5 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -277,8 +277,8 @@ CreateConstraintEntry(const char *constraintName, add_exact_object_address(&domobject, addrs_auto); } - record_object_address_dependencies(&conobject, addrs_auto, - DEPENDENCY_AUTO); + lock_record_object_address_dependencies(&conobject, addrs_auto, + DEPENDENCY_AUTO); free_object_addresses(addrs_auto); /* Handle set of normal dependencies */ @@ -352,8 +352,8 @@ CreateConstraintEntry(const char *constraintName, } } - record_object_address_dependencies(&conobject, addrs_normal, - DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&conobject, addrs_normal, + DEPENDENCY_NORMAL); free_object_addresses(addrs_normal); /* @@ -858,9 +858,11 @@ ConstraintSetParentConstraint(Oid childConstrId, ObjectAddressSet(depender, ConstraintRelationId, childConstrId); ObjectAddressSet(referenced, ConstraintRelationId, parentConstrId); + depLockAndCheckObject(&referenced); recordDependencyOn(&depender, &referenced, DEPENDENCY_PARTITION_PRI); ObjectAddressSet(referenced, RelationRelationId, childTableId); + depLockAndCheckObject(&referenced); recordDependencyOn(&depender, &referenced, DEPENDENCY_PARTITION_SEC); } else diff --git a/src/backend/catalog/pg_conversion.c b/src/backend/catalog/pg_conversion.c index 0770878eac..479dd528eb 100644 --- a/src/backend/catalog/pg_conversion.c +++ b/src/backend/catalog/pg_conversion.c @@ -116,12 +116,14 @@ ConversionCreate(const char *conname, Oid connamespace, referenced.classId = ProcedureRelationId; referenced.objectId = conproc; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); /* create dependency on namespace */ referenced.classId = NamespaceRelationId; referenced.objectId = connamespace; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); /* create dependency on owner */ diff --git a/src/backend/catalog/pg_depend.c b/src/backend/catalog/pg_depend.c index cfd7ef51df..47968ee8c8 100644 --- a/src/backend/catalog/pg_depend.c +++ b/src/backend/catalog/pg_depend.c @@ -20,21 +20,20 @@ #include "catalog/catalog.h" #include "catalog/dependency.h" #include "catalog/indexing.h" +#include "catalog/pg_auth_members.h" #include "catalog/pg_constraint.h" #include "catalog/pg_depend.h" #include "catalog/pg_extension.h" #include "catalog/partition.h" #include "commands/extension.h" #include "miscadmin.h" +#include "storage/lock.h" #include "utils/fmgroids.h" #include "utils/lsyscache.h" #include "utils/syscache.h" #include "utils/rel.h" -static bool isObjectPinned(const ObjectAddress *object); - - /* * Record a dependency between 2 objects via their respective objectAddress. * The first argument is the dependent object, the second the one it @@ -100,6 +99,25 @@ recordMultipleDependencies(const ObjectAddress *depender, slot_init_count = 0; for (i = 0; i < nreferenced; i++, referenced++) { +#ifdef USE_ASSERT_CHECKING + LOCKTAG tag; + + SET_LOCKTAG_OBJECT(tag, + MyDatabaseId, + referenced->classId, + referenced->objectId, + 0); + + /* + * Assert the referenced object is locked (see + * depLockAndCheckObject()) when recording the dependency. + */ + Assert(isObjectPinned(referenced) || + referenced->classId == RelationRelationId || + referenced->classId == AuthMemRelationId || + LockHeldByMe(&tag, AccessShareLock)); +#endif + /* * If the referenced object is pinned by the system, there's no real * need to record dependencies on it. This saves lots of space in @@ -239,6 +257,7 @@ recordDependencyOnCurrentExtension(const ObjectAddress *object, extension.objectId = CurrentExtensionObject; extension.objectSubId = 0; + depLockAndCheckObject(&extension); recordDependencyOn(object, &extension, DEPENDENCY_EXTENSION); } } @@ -501,11 +520,18 @@ changeDependencyFor(Oid classId, Oid objectId, depAddr.classId = classId; depAddr.objectId = objectId; depAddr.objectSubId = 0; + depLockAndCheckObject(&objAddr); recordDependencyOn(&depAddr, &objAddr, DEPENDENCY_NORMAL); return 1; } + /* + * Acquire a lock and check object still exists while changing the + * dependency. + */ + depLockAndCheckObject(&objAddr); + depRel = table_open(DependRelationId, RowExclusiveLock); /* There should be existing dependency record(s), so search. */ @@ -706,7 +732,7 @@ changeDependenciesOn(Oid refClassId, Oid oldRefObjectId, * The passed subId, if any, is ignored; we assume that only whole objects * are pinned (and that this implies pinning their components). */ -static bool +bool isObjectPinned(const ObjectAddress *object) { return IsPinnedObject(object->classId, object->objectId); diff --git a/src/backend/catalog/pg_operator.c b/src/backend/catalog/pg_operator.c index 65b45a424a..7e40471baf 100644 --- a/src/backend/catalog/pg_operator.c +++ b/src/backend/catalog/pg_operator.c @@ -930,7 +930,7 @@ makeOperatorDependencies(HeapTuple tuple, add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); /* Dependency on owner */ diff --git a/src/backend/catalog/pg_proc.c b/src/backend/catalog/pg_proc.c index 528c17cd7f..aa36a07648 100644 --- a/src/backend/catalog/pg_proc.c +++ b/src/backend/catalog/pg_proc.c @@ -593,6 +593,13 @@ ProcedureCreate(const char *procedureName, if (is_update) deleteDependencyRecordsFor(ProcedureRelationId, retval, true); + /* + * CommandCounterIncrement here to ensure the new function entry is + * visible when we'll check of object existence when recording the + * dependencies. + */ + CommandCounterIncrement(); + addrs = new_object_addresses(); ObjectAddressSet(myself, ProcedureRelationId, retval); @@ -637,7 +644,7 @@ ProcedureCreate(const char *procedureName, add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); /* dependency on SQL routine body */ @@ -674,9 +681,6 @@ ProcedureCreate(const char *procedureName, ArrayType *set_items = NULL; int save_nestlevel = 0; - /* Advance command counter so new tuple can be seen by validator */ - CommandCounterIncrement(); - /* * Set per-function configuration parameters so that the validation is * done with the environment the function expects. However, if diff --git a/src/backend/catalog/pg_publication.c b/src/backend/catalog/pg_publication.c index 0602398a54..6654b65f41 100644 --- a/src/backend/catalog/pg_publication.c +++ b/src/backend/catalog/pg_publication.c @@ -438,10 +438,12 @@ publication_add_relation(Oid pubid, PublicationRelInfo *pri, /* Add dependency on the publication */ ObjectAddressSet(referenced, PublicationRelationId, pubid); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_AUTO); /* Add dependency on the relation */ ObjectAddressSet(referenced, RelationRelationId, relid); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_AUTO); /* Add dependency on the objects mentioned in the qualifications */ @@ -454,6 +456,7 @@ publication_add_relation(Oid pubid, PublicationRelInfo *pri, for (int i = 0; i < natts; i++) { ObjectAddressSubSet(referenced, RelationRelationId, relid, attarray[i]); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); } @@ -661,10 +664,12 @@ publication_add_schema(Oid pubid, Oid schemaid, bool if_not_exists) /* Add dependency on the publication */ ObjectAddressSet(referenced, PublicationRelationId, pubid); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_AUTO); /* Add dependency on the schema */ ObjectAddressSet(referenced, NamespaceRelationId, schemaid); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_AUTO); /* Close the table */ diff --git a/src/backend/catalog/pg_range.c b/src/backend/catalog/pg_range.c index 501a6ba410..a2f343a81a 100644 --- a/src/backend/catalog/pg_range.c +++ b/src/backend/catalog/pg_range.c @@ -92,13 +92,14 @@ RangeCreate(Oid rangeTypeOid, Oid rangeSubType, Oid rangeCollation, add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); /* record multirange type's dependency on the range type */ referencing.classId = TypeRelationId; referencing.objectId = multirangeTypeOid; referencing.objectSubId = 0; + depLockAndCheckObject(&myself); recordDependencyOn(&referencing, &myself, DEPENDENCY_INTERNAL); table_close(pg_range, RowExclusiveLock); diff --git a/src/backend/catalog/pg_type.c b/src/backend/catalog/pg_type.c index 395dec8ed8..91139e275f 100644 --- a/src/backend/catalog/pg_type.c +++ b/src/backend/catalog/pg_type.c @@ -494,6 +494,14 @@ TypeCreate(Oid newTypeOid, * Create dependencies. We can/must skip this in bootstrap mode. */ if (!IsBootstrapProcessingMode()) + { + /* + * CommandCounterIncrement here to ensure the new type entry is + * visible when we'll check of object existence when recording the + * dependencies. + */ + CommandCounterIncrement(); + GenerateTypeDependencies(tup, pg_type_desc, (defaultTypeBin ? @@ -505,6 +513,7 @@ TypeCreate(Oid newTypeOid, isDependentType, true, /* make extension dependency */ rebuildDeps); + } /* Post creation hook for new type */ InvokeObjectPostCreateHook(TypeRelationId, typeObjectId, 0); @@ -703,7 +712,7 @@ GenerateTypeDependencies(HeapTuple typeTuple, add_exact_object_address(&referenced, addrs_normal); } - record_object_address_dependencies(&myself, addrs_normal, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs_normal, DEPENDENCY_NORMAL); free_object_addresses(addrs_normal); /* Normal dependency on the default expression. */ @@ -724,9 +733,15 @@ GenerateTypeDependencies(HeapTuple typeTuple, ObjectAddressSet(referenced, RelationRelationId, typeForm->typrelid); if (relationKind != RELKIND_COMPOSITE_TYPE) + { + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_INTERNAL); + } else + { + depLockAndCheckObject(&myself); recordDependencyOn(&referenced, &myself, DEPENDENCY_INTERNAL); + } } /* @@ -737,6 +752,7 @@ GenerateTypeDependencies(HeapTuple typeTuple, if (OidIsValid(typeForm->typelem)) { ObjectAddressSet(referenced, TypeRelationId, typeForm->typelem); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, isImplicitArray ? DEPENDENCY_INTERNAL : DEPENDENCY_NORMAL); } diff --git a/src/backend/catalog/toasting.c b/src/backend/catalog/toasting.c index 738bc46ae8..a471833efe 100644 --- a/src/backend/catalog/toasting.c +++ b/src/backend/catalog/toasting.c @@ -367,6 +367,7 @@ create_toast_table(Relation rel, Oid toastOid, Oid toastIndexOid, toastobject.objectId = toast_relid; toastobject.objectSubId = 0; + depLockAndCheckObject(&baseobject); recordDependencyOn(&toastobject, &baseobject, DEPENDENCY_INTERNAL); } diff --git a/src/backend/commands/alter.c b/src/backend/commands/alter.c index 4f99ebb447..e3ad5db529 100644 --- a/src/backend/commands/alter.c +++ b/src/backend/commands/alter.c @@ -501,7 +501,10 @@ ExecAlterObjectDependsStmt(AlterObjectDependsStmt *stmt, ObjectAddress *refAddre currexts = getAutoExtensionsOfObject(address.classId, address.objectId); if (!list_member_oid(currexts, refAddr.objectId)) + { + depLockAndCheckObject(&refAddr); recordDependencyOn(&address, &refAddr, DEPENDENCY_AUTO_EXTENSION); + } } return address; diff --git a/src/backend/commands/amcmds.c b/src/backend/commands/amcmds.c index aaa0f9a1dc..0b4e44131a 100644 --- a/src/backend/commands/amcmds.c +++ b/src/backend/commands/amcmds.c @@ -104,6 +104,7 @@ CreateAccessMethod(CreateAmStmt *stmt) referenced.objectId = amhandler; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); recordDependencyOnCurrentExtension(&myself, false); diff --git a/src/backend/commands/cluster.c b/src/backend/commands/cluster.c index 78f96789b0..73bcd68a1f 100644 --- a/src/backend/commands/cluster.c +++ b/src/backend/commands/cluster.c @@ -1381,6 +1381,7 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class, { baseobject.objectId = r1; toastobject.objectId = relform1->reltoastrelid; + depLockAndCheckObject(&baseobject); recordDependencyOn(&toastobject, &baseobject, DEPENDENCY_INTERNAL); } @@ -1389,6 +1390,7 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class, { baseobject.objectId = r2; toastobject.objectId = relform2->reltoastrelid; + depLockAndCheckObject(&baseobject); recordDependencyOn(&toastobject, &baseobject, DEPENDENCY_INTERNAL); } diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c index 7a5ed6b985..ddcba91c92 100644 --- a/src/backend/commands/event_trigger.c +++ b/src/backend/commands/event_trigger.c @@ -327,6 +327,7 @@ insert_event_trigger_tuple(const char *trigname, const char *eventname, Oid evtO referenced.classId = ProcedureRelationId; referenced.objectId = funcoid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); /* Depend on extension, if any. */ diff --git a/src/backend/commands/extension.c b/src/backend/commands/extension.c index 1643c8c69a..fdcffcea01 100644 --- a/src/backend/commands/extension.c +++ b/src/backend/commands/extension.c @@ -1935,7 +1935,7 @@ InsertExtensionTuple(const char *extName, Oid extOwner, } /* Record all of them (this includes duplicate elimination) */ - record_object_address_dependencies(&myself, refobjs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, refobjs, DEPENDENCY_NORMAL); free_object_addresses(refobjs); /* Post creation hook for new extension */ @@ -3258,6 +3258,7 @@ ApplyExtensionUpdates(Oid extensionOid, otherext.objectId = reqext; otherext.objectSubId = 0; + depLockAndCheckObject(&otherext); recordDependencyOn(&myself, &otherext, DEPENDENCY_NORMAL); } @@ -3414,6 +3415,7 @@ ExecAlterExtensionContentsRecurse(AlterExtensionContentsStmt *stmt, /* * OK, add the dependency. */ + depLockAndCheckObject(&extension); recordDependencyOn(&object, &extension, DEPENDENCY_EXTENSION); /* diff --git a/src/backend/commands/foreigncmds.c b/src/backend/commands/foreigncmds.c index cf61bbac1f..5e98f3d0d6 100644 --- a/src/backend/commands/foreigncmds.c +++ b/src/backend/commands/foreigncmds.c @@ -642,6 +642,7 @@ CreateForeignDataWrapper(ParseState *pstate, CreateFdwStmt *stmt) referenced.classId = ProcedureRelationId; referenced.objectId = fdwhandler; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); } @@ -650,6 +651,7 @@ CreateForeignDataWrapper(ParseState *pstate, CreateFdwStmt *stmt) referenced.classId = ProcedureRelationId; referenced.objectId = fdwvalidator; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); } @@ -811,6 +813,7 @@ AlterForeignDataWrapper(ParseState *pstate, AlterFdwStmt *stmt) referenced.classId = ProcedureRelationId; referenced.objectId = fdwhandler; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); } @@ -819,6 +822,7 @@ AlterForeignDataWrapper(ParseState *pstate, AlterFdwStmt *stmt) referenced.classId = ProcedureRelationId; referenced.objectId = fdwvalidator; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); } } @@ -951,6 +955,7 @@ CreateForeignServer(CreateForeignServerStmt *stmt) referenced.classId = ForeignDataWrapperRelationId; referenced.objectId = fdw->fdwid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); recordDependencyOnOwner(ForeignServerRelationId, srvId, ownerId); @@ -1195,6 +1200,7 @@ CreateUserMapping(CreateUserMappingStmt *stmt) referenced.classId = ForeignServerRelationId; referenced.objectId = srv->serverid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); if (OidIsValid(useId)) @@ -1472,6 +1478,7 @@ CreateForeignTable(CreateForeignTableStmt *stmt, Oid relid) referenced.classId = ForeignServerRelationId; referenced.objectId = server->serverid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); table_close(ftrel, RowExclusiveLock); diff --git a/src/backend/commands/functioncmds.c b/src/backend/commands/functioncmds.c index 6593fd7d81..2a378a53d8 100644 --- a/src/backend/commands/functioncmds.c +++ b/src/backend/commands/functioncmds.c @@ -1459,6 +1459,7 @@ AlterFunction(ParseState *pstate, AlterFunctionStmt *stmt) referenced.classId = ProcedureRelationId; referenced.objectId = newsupport; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&address, &referenced, DEPENDENCY_NORMAL); } @@ -1979,7 +1980,7 @@ CreateTransform(CreateTransformStmt *stmt) add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); /* dependency on extension */ diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c index 309389e20d..487708d885 100644 --- a/src/backend/commands/indexcmds.c +++ b/src/backend/commands/indexcmds.c @@ -4377,8 +4377,10 @@ IndexSetParentIndex(Relation partitionIdx, Oid parentOid) ObjectAddressSet(parentIdx, RelationRelationId, parentOid); ObjectAddressSet(partitionTbl, RelationRelationId, partitionIdx->rd_index->indrelid); + depLockAndCheckObject(&parentIdx); recordDependencyOn(&partIdx, &parentIdx, DEPENDENCY_PARTITION_PRI); + depLockAndCheckObject(&partitionTbl); recordDependencyOn(&partIdx, &partitionTbl, DEPENDENCY_PARTITION_SEC); } diff --git a/src/backend/commands/opclasscmds.c b/src/backend/commands/opclasscmds.c index b8b5c147c5..e7b1cb911e 100644 --- a/src/backend/commands/opclasscmds.c +++ b/src/backend/commands/opclasscmds.c @@ -298,12 +298,14 @@ CreateOpFamily(CreateOpFamilyStmt *stmt, const char *opfname, referenced.classId = AccessMethodRelationId; referenced.objectId = amoid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_AUTO); /* dependency on namespace */ referenced.classId = NamespaceRelationId; referenced.objectId = namespaceoid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); /* dependency on owner */ @@ -725,18 +727,21 @@ DefineOpClass(CreateOpClassStmt *stmt) referenced.classId = NamespaceRelationId; referenced.objectId = namespaceoid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); /* dependency on opfamily */ referenced.classId = OperatorFamilyRelationId; referenced.objectId = opfamilyoid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_AUTO); /* dependency on indexed datatype */ referenced.classId = TypeRelationId; referenced.objectId = typeoid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); /* dependency on storage datatype */ @@ -745,6 +750,7 @@ DefineOpClass(CreateOpClassStmt *stmt) referenced.classId = TypeRelationId; referenced.objectId = storageoid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); } @@ -1486,6 +1492,13 @@ storeOperators(List *opfamilyname, Oid amoid, Oid opfamilyoid, heap_freetuple(tup); + /* + * CommandCounterIncrement here to ensure the new operator entry is + * visible when we'll check of object existence when recording the + * dependencies. + */ + CommandCounterIncrement(); + /* Make its dependencies */ myself.classId = AccessMethodOperatorRelationId; myself.objectId = entryoid; @@ -1496,6 +1509,7 @@ storeOperators(List *opfamilyname, Oid amoid, Oid opfamilyoid, referenced.objectSubId = 0; /* see comments in amapi.h about dependency strength */ + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, op->ref_is_hard ? DEPENDENCY_NORMAL : DEPENDENCY_AUTO); @@ -1504,6 +1518,7 @@ storeOperators(List *opfamilyname, Oid amoid, Oid opfamilyoid, referenced.objectId = op->refobjid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, op->ref_is_hard ? DEPENDENCY_INTERNAL : DEPENDENCY_AUTO); @@ -1514,6 +1529,7 @@ storeOperators(List *opfamilyname, Oid amoid, Oid opfamilyoid, referenced.objectId = op->sortfamily; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, op->ref_is_hard ? DEPENDENCY_NORMAL : DEPENDENCY_AUTO); } @@ -1597,6 +1613,7 @@ storeProcedures(List *opfamilyname, Oid amoid, Oid opfamilyoid, referenced.objectSubId = 0; /* see comments in amapi.h about dependency strength */ + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, proc->ref_is_hard ? DEPENDENCY_NORMAL : DEPENDENCY_AUTO); @@ -1605,6 +1622,7 @@ storeProcedures(List *opfamilyname, Oid amoid, Oid opfamilyoid, referenced.objectId = proc->refobjid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, proc->ref_is_hard ? DEPENDENCY_INTERNAL : DEPENDENCY_AUTO); diff --git a/src/backend/commands/policy.c b/src/backend/commands/policy.c index 6ff3eba824..5a50ad73fb 100644 --- a/src/backend/commands/policy.c +++ b/src/backend/commands/policy.c @@ -722,6 +722,7 @@ CreatePolicy(CreatePolicyStmt *stmt) myself.objectId = policy_id; myself.objectSubId = 0; + depLockAndCheckObject(&target); recordDependencyOn(&myself, &target, DEPENDENCY_AUTO); recordDependencyOnExpr(&myself, qual, qual_pstate->p_rtable, @@ -1053,6 +1054,7 @@ AlterPolicy(AlterPolicyStmt *stmt) myself.objectId = policy_id; myself.objectSubId = 0; + depLockAndCheckObject(&target); recordDependencyOn(&myself, &target, DEPENDENCY_AUTO); recordDependencyOnExpr(&myself, qual, qual_parse_rtable, DEPENDENCY_NORMAL); diff --git a/src/backend/commands/proclang.c b/src/backend/commands/proclang.c index 881f90017e..679155162e 100644 --- a/src/backend/commands/proclang.c +++ b/src/backend/commands/proclang.c @@ -205,7 +205,7 @@ CreateProceduralLanguage(CreatePLangStmt *stmt) add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); /* Post creation hook for new procedural language */ diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c index 28f8522264..0d37576d22 100644 --- a/src/backend/commands/sequence.c +++ b/src/backend/commands/sequence.c @@ -1681,6 +1681,7 @@ process_owned_by(Relation seqrel, List *owned_by, bool for_identity) depobject.classId = RelationRelationId; depobject.objectId = RelationGetRelid(seqrel); depobject.objectSubId = 0; + depLockAndCheckObject(&refobject); recordDependencyOn(&depobject, &refobject, deptype); } diff --git a/src/backend/commands/statscmds.c b/src/backend/commands/statscmds.c index 1db3ef69d2..455ec046f8 100644 --- a/src/backend/commands/statscmds.c +++ b/src/backend/commands/statscmds.c @@ -536,6 +536,7 @@ CreateStatistics(CreateStatsStmt *stmt) for (i = 0; i < nattnums; i++) { ObjectAddressSubSet(parentobject, RelationRelationId, relid, attnums[i]); + depLockAndCheckObject(&parentobject); recordDependencyOn(&myself, &parentobject, DEPENDENCY_AUTO); } @@ -553,6 +554,7 @@ CreateStatistics(CreateStatsStmt *stmt) if (!nattnums) { ObjectAddressSet(parentobject, RelationRelationId, relid); + depLockAndCheckObject(&parentobject); recordDependencyOn(&myself, &parentobject, DEPENDENCY_AUTO); } @@ -573,6 +575,7 @@ CreateStatistics(CreateStatsStmt *stmt) * than the underlying table(s). */ ObjectAddressSet(parentobject, NamespaceRelationId, namespaceId); + depLockAndCheckObject(&parentobject); recordDependencyOn(&myself, &parentobject, DEPENDENCY_NORMAL); recordDependencyOnOwner(StatisticExtRelationId, statoid, stxowner); diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 7b6c69b7a5..73696a4210 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -3437,6 +3437,7 @@ StoreCatalogInheritance1(Oid relationId, Oid parentOid, childobject.objectId = relationId; childobject.objectSubId = 0; + depLockAndCheckObject(&parentobject); recordDependencyOn(&childobject, &parentobject, child_dependency_type(child_is_partition)); @@ -7440,6 +7441,7 @@ add_column_datatype_dependency(Oid relid, int32 attnum, Oid typid) referenced.classId = TypeRelationId; referenced.objectId = typid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); } @@ -7461,6 +7463,7 @@ add_column_collation_dependency(Oid relid, int32 attnum, Oid collid) referenced.classId = CollationRelationId; referenced.objectId = collid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); } } @@ -10140,6 +10143,7 @@ addFkRecurseReferenced(List **wqueue, Constraint *fkconstraint, Relation rel, ObjectAddress referenced; ObjectAddressSet(referenced, ConstraintRelationId, parentConstr); + depLockAndCheckObject(&referenced); recordDependencyOn(&address, &referenced, DEPENDENCY_INTERNAL); } @@ -10431,8 +10435,10 @@ addFkRecurseReferencing(List **wqueue, Constraint *fkconstraint, Relation rel, */ ObjectAddressSet(address, ConstraintRelationId, constrOid); ObjectAddressSet(referenced, ConstraintRelationId, parentConstr); + depLockAndCheckObject(&referenced); recordDependencyOn(&address, &referenced, DEPENDENCY_PARTITION_PRI); ObjectAddressSet(referenced, RelationRelationId, partitionId); + depLockAndCheckObject(&referenced); recordDependencyOn(&address, &referenced, DEPENDENCY_PARTITION_SEC); /* Make all this visible before recursing */ @@ -10933,9 +10939,11 @@ CloneFkReferencing(List **wqueue, Relation parentRel, Relation partRel) /* Set up partition dependencies for the new constraint */ ObjectAddressSet(address, ConstraintRelationId, constrOid); ObjectAddressSet(referenced, ConstraintRelationId, parentConstrOid); + depLockAndCheckObject(&referenced); recordDependencyOn(&address, &referenced, DEPENDENCY_PARTITION_PRI); ObjectAddressSet(referenced, RelationRelationId, RelationGetRelid(partRel)); + depLockAndCheckObject(&referenced); recordDependencyOn(&address, &referenced, DEPENDENCY_PARTITION_SEC); /* Done with the cloned constraint's tuple */ @@ -14782,6 +14790,7 @@ ATExecSetAccessMethodNoStorage(Relation rel, Oid newAccessMethodId) */ ObjectAddressSet(relobj, RelationRelationId, reloid); ObjectAddressSet(referenced, AccessMethodRelationId, rd_rel->relam); + depLockAndCheckObject(&referenced); recordDependencyOn(&relobj, &referenced, DEPENDENCY_NORMAL); } else if (OidIsValid(oldAccessMethodId) && @@ -16400,6 +16409,7 @@ ATExecAddOf(Relation rel, const TypeName *ofTypename, LOCKMODE lockmode) typeobj.classId = TypeRelationId; typeobj.objectId = typeid; typeobj.objectSubId = 0; + depLockAndCheckObject(&typeobj); recordDependencyOn(&tableobj, &typeobj, DEPENDENCY_NORMAL); /* Update pg_class.reloftype */ diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c index 95de402fa6..c0e7ddff1c 100644 --- a/src/backend/commands/trigger.c +++ b/src/backend/commands/trigger.c @@ -1018,8 +1018,6 @@ CreateTriggerFiringOn(CreateTrigStmt *stmt, const char *queryString, ((Form_pg_class) GETSTRUCT(tuple))->relhastriggers = true; CatalogTupleUpdate(pgrel, &tuple->t_self, tuple); - - CommandCounterIncrement(); } else CacheInvalidateRelcacheByTuple(tuple); @@ -1027,6 +1025,12 @@ CreateTriggerFiringOn(CreateTrigStmt *stmt, const char *queryString, heap_freetuple(tuple); table_close(pgrel, RowExclusiveLock); + /* + * CommandCounterIncrement here to ensure the new trigger entry is visible + * when we'll check of object existence when recording the dependencies. + */ + CommandCounterIncrement(); + /* * If we're replacing a trigger, flush all the old dependencies before * recording new ones. @@ -1045,6 +1049,7 @@ CreateTriggerFiringOn(CreateTrigStmt *stmt, const char *queryString, referenced.classId = ProcedureRelationId; referenced.objectId = funcoid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); if (isInternal && OidIsValid(constraintOid)) @@ -1058,6 +1063,7 @@ CreateTriggerFiringOn(CreateTrigStmt *stmt, const char *queryString, referenced.classId = ConstraintRelationId; referenced.objectId = constraintOid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_INTERNAL); } else @@ -1070,6 +1076,7 @@ CreateTriggerFiringOn(CreateTrigStmt *stmt, const char *queryString, referenced.classId = RelationRelationId; referenced.objectId = RelationGetRelid(rel); referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_AUTO); if (OidIsValid(constrrelid)) @@ -1077,6 +1084,7 @@ CreateTriggerFiringOn(CreateTrigStmt *stmt, const char *queryString, referenced.classId = RelationRelationId; referenced.objectId = constrrelid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_AUTO); } /* Not possible to have an index dependency in this case */ @@ -1091,6 +1099,7 @@ CreateTriggerFiringOn(CreateTrigStmt *stmt, const char *queryString, referenced.classId = ConstraintRelationId; referenced.objectId = constraintOid; referenced.objectSubId = 0; + depLockAndCheckObject(&myself); recordDependencyOn(&referenced, &myself, DEPENDENCY_INTERNAL); } @@ -1100,8 +1109,10 @@ CreateTriggerFiringOn(CreateTrigStmt *stmt, const char *queryString, if (OidIsValid(parentTriggerOid)) { ObjectAddressSet(referenced, TriggerRelationId, parentTriggerOid); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_PARTITION_PRI); ObjectAddressSet(referenced, RelationRelationId, RelationGetRelid(rel)); + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_PARTITION_SEC); } } @@ -1116,6 +1127,7 @@ CreateTriggerFiringOn(CreateTrigStmt *stmt, const char *queryString, for (i = 0; i < ncolumns; i++) { referenced.objectSubId = columns[i]; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL); } } @@ -1255,9 +1267,11 @@ TriggerSetParentTrigger(Relation trigRel, ObjectAddressSet(depender, TriggerRelationId, childTrigId); ObjectAddressSet(referenced, TriggerRelationId, parentTrigId); + depLockAndCheckObject(&referenced); recordDependencyOn(&depender, &referenced, DEPENDENCY_PARTITION_PRI); ObjectAddressSet(referenced, RelationRelationId, childTableId); + depLockAndCheckObject(&referenced); recordDependencyOn(&depender, &referenced, DEPENDENCY_PARTITION_SEC); } else diff --git a/src/backend/commands/tsearchcmds.c b/src/backend/commands/tsearchcmds.c index b7b5019f1e..a62c997725 100644 --- a/src/backend/commands/tsearchcmds.c +++ b/src/backend/commands/tsearchcmds.c @@ -171,7 +171,7 @@ makeParserDependencies(HeapTuple tuple) add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); return myself; @@ -329,7 +329,7 @@ makeDictionaryDependencies(HeapTuple tuple) ObjectAddressSet(referenced, TSTemplateRelationId, dict->dicttemplate); add_exact_object_address(&referenced, addrs); - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); return myself; @@ -677,7 +677,7 @@ makeTSTemplateDependencies(HeapTuple tuple) add_exact_object_address(&referenced, addrs); } - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); return myself; @@ -885,7 +885,7 @@ makeConfigurationDependencies(HeapTuple tuple, bool removeOld, } /* Record 'em (this includes duplicate elimination) */ - record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); + lock_record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL); free_object_addresses(addrs); diff --git a/src/backend/commands/typecmds.c b/src/backend/commands/typecmds.c index 2a1e713335..cbb61c1207 100644 --- a/src/backend/commands/typecmds.c +++ b/src/backend/commands/typecmds.c @@ -1794,6 +1794,7 @@ makeRangeConstructors(const char *name, Oid namespace, * that they go away silently when the type is dropped. Note that * pg_dump depends on this choice to avoid dumping the constructors. */ + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_INTERNAL); } } @@ -1859,6 +1860,7 @@ makeMultirangeConstructors(const char *name, Oid namespace, * that they go away silently when the type is dropped. Note that pg_dump * depends on this choice to avoid dumping the constructors. */ + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_INTERNAL); pfree(argtypes); @@ -1898,6 +1900,7 @@ makeMultirangeConstructors(const char *name, Oid namespace, 1.0, /* procost */ 0.0); /* prorows */ /* ditto */ + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_INTERNAL); pfree(argtypes); *castFuncOid = myself.objectId; @@ -1936,6 +1939,7 @@ makeMultirangeConstructors(const char *name, Oid namespace, 1.0, /* procost */ 0.0); /* prorows */ /* ditto */ + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, DEPENDENCY_INTERNAL); pfree(argtypes); pfree(allParameterTypes); diff --git a/src/backend/rewrite/rewriteDefine.c b/src/backend/rewrite/rewriteDefine.c index 6cc9a8d8bf..53308dceb6 100644 --- a/src/backend/rewrite/rewriteDefine.c +++ b/src/backend/rewrite/rewriteDefine.c @@ -155,6 +155,7 @@ InsertRule(const char *rulname, referenced.objectId = eventrel_oid; referenced.objectSubId = 0; + depLockAndCheckObject(&referenced); recordDependencyOn(&myself, &referenced, (evtype == CMD_SELECT) ? DEPENDENCY_INTERNAL : DEPENDENCY_AUTO); diff --git a/src/backend/utils/errcodes.txt b/src/backend/utils/errcodes.txt index 3250d539e1..60e8539fe3 100644 --- a/src/backend/utils/errcodes.txt +++ b/src/backend/utils/errcodes.txt @@ -271,6 +271,7 @@ Section: Class 28 - Invalid Authorization Specification Section: Class 2B - Dependent Privilege Descriptors Still Exist 2B000 E ERRCODE_DEPENDENT_PRIVILEGE_DESCRIPTORS_STILL_EXIST dependent_privilege_descriptors_still_exist +2BP02 E ERRCODE_DEPENDENT_OBJECTS_DOES_NOT_EXIST dependent_objects_does_not_exist 2BP01 E ERRCODE_DEPENDENT_OBJECTS_STILL_EXIST dependent_objects_still_exist Section: Class 2D - Invalid Transaction Termination diff --git a/src/include/catalog/dependency.h b/src/include/catalog/dependency.h index 7eee66f810..f404a92fb4 100644 --- a/src/include/catalog/dependency.h +++ b/src/include/catalog/dependency.h @@ -101,6 +101,8 @@ typedef struct ObjectAddresses ObjectAddresses; /* in dependency.c */ extern void AcquireDeletionLock(const ObjectAddress *object, int flags); +extern void depLockAndCheckObject(const ObjectAddress *object); +extern void depLockAndCheckObjects(const ObjectAddress *object, int nobject); extern void ReleaseDeletionLock(const ObjectAddress *object); @@ -128,9 +130,9 @@ extern void add_exact_object_address(const ObjectAddress *object, extern bool object_address_present(const ObjectAddress *object, const ObjectAddresses *addrs); -extern void record_object_address_dependencies(const ObjectAddress *depender, - ObjectAddresses *referenced, - DependencyType behavior); +extern void lock_record_object_address_dependencies(const ObjectAddress *depender, + ObjectAddresses *referenced, + DependencyType behavior); extern void sort_object_addresses(ObjectAddresses *addrs); @@ -172,6 +174,7 @@ extern long changeDependenciesOf(Oid classId, Oid oldObjectId, extern long changeDependenciesOn(Oid refClassId, Oid oldRefObjectId, Oid newRefObjectId); +extern bool isObjectPinned(const ObjectAddress *object); extern Oid getExtensionOfObject(Oid classId, Oid objectId); extern List *getAutoExtensionsOfObject(Oid classId, Oid objectId); diff --git a/src/include/catalog/objectaddress.h b/src/include/catalog/objectaddress.h index 3a70d80e32..56f746264b 100644 --- a/src/include/catalog/objectaddress.h +++ b/src/include/catalog/objectaddress.h @@ -53,6 +53,7 @@ extern void check_object_ownership(Oid roleid, Node *object, Relation relation); extern Oid get_object_namespace(const ObjectAddress *address); +extern bool ObjectByIdExist(const ObjectAddress *address); extern bool is_objectclass_supported(Oid class_id); extern const char *get_object_class_descr(Oid class_id); diff --git a/src/test/isolation/expected/test_dependencies_locks.out b/src/test/isolation/expected/test_dependencies_locks.out new file mode 100644 index 0000000000..9b645d7aa5 --- /dev/null +++ b/src/test/isolation/expected/test_dependencies_locks.out @@ -0,0 +1,129 @@ +Parsed test spec with 2 sessions + +starting permutation: s1_begin s1_create_function_in_schema s2_drop_schema s1_commit +step s1_begin: BEGIN; +step s1_create_function_in_schema: CREATE FUNCTION testschema.foo() RETURNS int AS 'select 1' LANGUAGE sql; +step s2_drop_schema: DROP SCHEMA testschema; <waiting ...> +step s1_commit: COMMIT; +step s2_drop_schema: <... completed> +ERROR: cannot drop schema testschema because other objects depend on it + +starting permutation: s2_begin s2_drop_schema s1_create_function_in_schema s2_commit +step s2_begin: BEGIN; +step s2_drop_schema: DROP SCHEMA testschema; +step s1_create_function_in_schema: CREATE FUNCTION testschema.foo() RETURNS int AS 'select 1' LANGUAGE sql; <waiting ...> +step s2_commit: COMMIT; +step s1_create_function_in_schema: <... completed> +ERROR: schema testschema does not exist + +starting permutation: s1_begin s1_alter_function_schema s2_drop_alterschema s1_commit +step s1_begin: BEGIN; +step s1_alter_function_schema: ALTER FUNCTION public.falter() SET SCHEMA alterschema; +step s2_drop_alterschema: DROP SCHEMA alterschema; <waiting ...> +step s1_commit: COMMIT; +step s2_drop_alterschema: <... completed> +ERROR: cannot drop schema alterschema because other objects depend on it + +starting permutation: s2_begin s2_drop_alterschema s1_alter_function_schema s2_commit +step s2_begin: BEGIN; +step s2_drop_alterschema: DROP SCHEMA alterschema; +step s1_alter_function_schema: ALTER FUNCTION public.falter() SET SCHEMA alterschema; <waiting ...> +step s2_commit: COMMIT; +step s1_alter_function_schema: <... completed> +ERROR: schema alterschema does not exist + +starting permutation: s1_begin s1_create_function_with_argtype s2_drop_foo_type s1_commit +step s1_begin: BEGIN; +step s1_create_function_with_argtype: CREATE FUNCTION fooargtype(num foo) RETURNS int AS 'select 1' LANGUAGE sql; +step s2_drop_foo_type: DROP TYPE public.foo; <waiting ...> +step s1_commit: COMMIT; +step s2_drop_foo_type: <... completed> +ERROR: cannot drop type foo because other objects depend on it + +starting permutation: s2_begin s2_drop_foo_type s1_create_function_with_argtype s2_commit +step s2_begin: BEGIN; +step s2_drop_foo_type: DROP TYPE public.foo; +step s1_create_function_with_argtype: CREATE FUNCTION fooargtype(num foo) RETURNS int AS 'select 1' LANGUAGE sql; <waiting ...> +step s2_commit: COMMIT; +step s1_create_function_with_argtype: <... completed> +ERROR: type foo does not exist + +starting permutation: s1_begin s1_create_function_with_rettype s2_drop_foo_rettype s1_commit +step s1_begin: BEGIN; +step s1_create_function_with_rettype: CREATE FUNCTION footrettype() RETURNS id LANGUAGE sql RETURN 1; +step s2_drop_foo_rettype: DROP DOMAIN id; <waiting ...> +step s1_commit: COMMIT; +step s2_drop_foo_rettype: <... completed> +ERROR: cannot drop type id because other objects depend on it + +starting permutation: s2_begin s2_drop_foo_rettype s1_create_function_with_rettype s2_commit +step s2_begin: BEGIN; +step s2_drop_foo_rettype: DROP DOMAIN id; +step s1_create_function_with_rettype: CREATE FUNCTION footrettype() RETURNS id LANGUAGE sql RETURN 1; <waiting ...> +step s2_commit: COMMIT; +step s1_create_function_with_rettype: <... completed> +ERROR: type id does not exist + +starting permutation: s1_begin s1_create_function_with_function s2_drop_function_f s1_commit +step s1_begin: BEGIN; +step s1_create_function_with_function: CREATE FUNCTION foofunc() RETURNS int LANGUAGE SQL RETURN f() + 1; +step s2_drop_function_f: DROP FUNCTION f(); <waiting ...> +step s1_commit: COMMIT; +step s2_drop_function_f: <... completed> +ERROR: cannot drop function f() because other objects depend on it + +starting permutation: s2_begin s2_drop_function_f s1_create_function_with_function s2_commit +step s2_begin: BEGIN; +step s2_drop_function_f: DROP FUNCTION f(); +step s1_create_function_with_function: CREATE FUNCTION foofunc() RETURNS int LANGUAGE SQL RETURN f() + 1; <waiting ...> +step s2_commit: COMMIT; +step s1_create_function_with_function: <... completed> +ERROR: function f() does not exist + +starting permutation: s1_begin s1_create_domain_with_domain s2_drop_domain_id s1_commit +step s1_begin: BEGIN; +step s1_create_domain_with_domain: CREATE DOMAIN idid as id; +step s2_drop_domain_id: DROP DOMAIN id; <waiting ...> +step s1_commit: COMMIT; +step s2_drop_domain_id: <... completed> +ERROR: cannot drop type id because other objects depend on it + +starting permutation: s2_begin s2_drop_domain_id s1_create_domain_with_domain s2_commit +step s2_begin: BEGIN; +step s2_drop_domain_id: DROP DOMAIN id; +step s1_create_domain_with_domain: CREATE DOMAIN idid as id; <waiting ...> +step s2_commit: COMMIT; +step s1_create_domain_with_domain: <... completed> +ERROR: type id does not exist + +starting permutation: s1_begin s1_create_table_with_type s2_drop_footab_type s1_commit +step s1_begin: BEGIN; +step s1_create_table_with_type: CREATE TABLE tabtype(a footab); +step s2_drop_footab_type: DROP TYPE public.footab; <waiting ...> +step s1_commit: COMMIT; +step s2_drop_footab_type: <... completed> +ERROR: cannot drop type footab because other objects depend on it + +starting permutation: s2_begin s2_drop_footab_type s1_create_table_with_type s2_commit +step s2_begin: BEGIN; +step s2_drop_footab_type: DROP TYPE public.footab; +step s1_create_table_with_type: CREATE TABLE tabtype(a footab); <waiting ...> +step s2_commit: COMMIT; +step s1_create_table_with_type: <... completed> +ERROR: type footab does not exist + +starting permutation: s1_begin s1_create_server_with_fdw_wrapper s2_drop_fdw_wrapper s1_commit +step s1_begin: BEGIN; +step s1_create_server_with_fdw_wrapper: CREATE SERVER srv_fdw_wrapper FOREIGN DATA WRAPPER fdw_wrapper; +step s2_drop_fdw_wrapper: DROP FOREIGN DATA WRAPPER fdw_wrapper RESTRICT; <waiting ...> +step s1_commit: COMMIT; +step s2_drop_fdw_wrapper: <... completed> +ERROR: cannot drop foreign-data wrapper fdw_wrapper because other objects depend on it + +starting permutation: s2_begin s2_drop_fdw_wrapper s1_create_server_with_fdw_wrapper s2_commit +step s2_begin: BEGIN; +step s2_drop_fdw_wrapper: DROP FOREIGN DATA WRAPPER fdw_wrapper RESTRICT; +step s1_create_server_with_fdw_wrapper: CREATE SERVER srv_fdw_wrapper FOREIGN DATA WRAPPER fdw_wrapper; <waiting ...> +step s2_commit: COMMIT; +step s1_create_server_with_fdw_wrapper: <... completed> +ERROR: foreign-data wrapper fdw_wrapper does not exist diff --git a/src/test/isolation/isolation_schedule b/src/test/isolation/isolation_schedule index 0342eb39e4..1b67f0bffe 100644 --- a/src/test/isolation/isolation_schedule +++ b/src/test/isolation/isolation_schedule @@ -114,3 +114,4 @@ test: serializable-parallel-2 test: serializable-parallel-3 test: matview-write-skew test: lock-nowait +test: test_dependencies_locks diff --git a/src/test/isolation/specs/test_dependencies_locks.spec b/src/test/isolation/specs/test_dependencies_locks.spec new file mode 100644 index 0000000000..5d04dfe9dc --- /dev/null +++ b/src/test/isolation/specs/test_dependencies_locks.spec @@ -0,0 +1,89 @@ +setup +{ + CREATE SCHEMA testschema; + CREATE SCHEMA alterschema; + CREATE TYPE public.foo as enum ('one', 'two'); + CREATE TYPE public.footab as enum ('three', 'four'); + CREATE DOMAIN id AS int; + CREATE FUNCTION f() RETURNS int LANGUAGE SQL RETURN 1; + CREATE FUNCTION public.falter() RETURNS int LANGUAGE SQL RETURN 1; + CREATE FOREIGN DATA WRAPPER fdw_wrapper; +} + +teardown +{ + DROP FUNCTION IF EXISTS testschema.foo(); + DROP FUNCTION IF EXISTS fooargtype(num foo); + DROP FUNCTION IF EXISTS footrettype(); + DROP FUNCTION IF EXISTS foofunc(); + DROP FUNCTION IF EXISTS public.falter(); + DROP FUNCTION IF EXISTS alterschema.falter(); + DROP DOMAIN IF EXISTS idid; + DROP SERVER IF EXISTS srv_fdw_wrapper; + DROP TABLE IF EXISTS tabtype; + DROP SCHEMA IF EXISTS testschema; + DROP SCHEMA IF EXISTS alterschema; + DROP TYPE IF EXISTS public.foo; + DROP TYPE IF EXISTS public.footab; + DROP DOMAIN IF EXISTS id; + DROP FUNCTION IF EXISTS f(); + DROP FOREIGN DATA WRAPPER IF EXISTS fdw_wrapper; +} + +session "s1" + +step "s1_begin" { BEGIN; } +step "s1_create_function_in_schema" { CREATE FUNCTION testschema.foo() RETURNS int AS 'select 1' LANGUAGE sql; } +step "s1_create_function_with_argtype" { CREATE FUNCTION fooargtype(num foo) RETURNS int AS 'select 1' LANGUAGE sql; } +step "s1_create_function_with_rettype" { CREATE FUNCTION footrettype() RETURNS id LANGUAGE sql RETURN 1; } +step "s1_create_function_with_function" { CREATE FUNCTION foofunc() RETURNS int LANGUAGE SQL RETURN f() + 1; } +step "s1_alter_function_schema" { ALTER FUNCTION public.falter() SET SCHEMA alterschema; } +step "s1_create_domain_with_domain" { CREATE DOMAIN idid as id; } +step "s1_create_table_with_type" { CREATE TABLE tabtype(a footab); } +step "s1_create_server_with_fdw_wrapper" { CREATE SERVER srv_fdw_wrapper FOREIGN DATA WRAPPER fdw_wrapper; } +step "s1_commit" { COMMIT; } + +session "s2" + +step "s2_begin" { BEGIN; } +step "s2_drop_schema" { DROP SCHEMA testschema; } +step "s2_drop_alterschema" { DROP SCHEMA alterschema; } +step "s2_drop_foo_type" { DROP TYPE public.foo; } +step "s2_drop_foo_rettype" { DROP DOMAIN id; } +step "s2_drop_footab_type" { DROP TYPE public.footab; } +step "s2_drop_function_f" { DROP FUNCTION f(); } +step "s2_drop_domain_id" { DROP DOMAIN id; } +step "s2_drop_fdw_wrapper" { DROP FOREIGN DATA WRAPPER fdw_wrapper RESTRICT; } +step "s2_commit" { COMMIT; } + +# function - schema +permutation "s1_begin" "s1_create_function_in_schema" "s2_drop_schema" "s1_commit" +permutation "s2_begin" "s2_drop_schema" "s1_create_function_in_schema" "s2_commit" + +# alter function - schema +permutation "s1_begin" "s1_alter_function_schema" "s2_drop_alterschema" "s1_commit" +permutation "s2_begin" "s2_drop_alterschema" "s1_alter_function_schema" "s2_commit" + +# function - argtype +permutation "s1_begin" "s1_create_function_with_argtype" "s2_drop_foo_type" "s1_commit" +permutation "s2_begin" "s2_drop_foo_type" "s1_create_function_with_argtype" "s2_commit" + +# function - rettype +permutation "s1_begin" "s1_create_function_with_rettype" "s2_drop_foo_rettype" "s1_commit" +permutation "s2_begin" "s2_drop_foo_rettype" "s1_create_function_with_rettype" "s2_commit" + +# function - function +permutation "s1_begin" "s1_create_function_with_function" "s2_drop_function_f" "s1_commit" +permutation "s2_begin" "s2_drop_function_f" "s1_create_function_with_function" "s2_commit" + +# domain - domain +permutation "s1_begin" "s1_create_domain_with_domain" "s2_drop_domain_id" "s1_commit" +permutation "s2_begin" "s2_drop_domain_id" "s1_create_domain_with_domain" "s2_commit" + +# table - type +permutation "s1_begin" "s1_create_table_with_type" "s2_drop_footab_type" "s1_commit" +permutation "s2_begin" "s2_drop_footab_type" "s1_create_table_with_type" "s2_commit" + +# server - foreign data wrapper +permutation "s1_begin" "s1_create_server_with_fdw_wrapper" "s2_drop_fdw_wrapper" "s1_commit" +permutation "s2_begin" "s2_drop_fdw_wrapper" "s1_create_server_with_fdw_wrapper" "s2_commit" -- 2.34.1 --xlZXO4tPcOr9Z9/W-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-16 23:28 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-16 23:28 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++++++ src/backend/commands/tablecmds.c | 46 ++++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 155 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 81 +++++++++++ 5 files changed, 339 insertions(+), 20 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..8f207c4a79d 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = (Var *) linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index e7386e81b07..005fbf78fbf 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8863,6 +8863,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8926,29 +8927,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 75f64628aef..4062f985c5a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,157 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4980,8 +5131,10 @@ alter table testgen.tpart alter column b add generated always as (a * 2) stored; ERROR: ALTER action ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED cannot be performed on relation "tpart" DETAIL: This operation is not supported for partitioned tables. drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects +NOTICE: drop cascades to 6 other objects DETAIL: drop cascades to table testgen.t3 +drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 drop cascades to table testgen.t2 drop cascades to table testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index d776595a6ed..821f04e8073 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3197,6 +3197,87 @@ alter table testgen.t3 alter column b add generated always as (a * 2) stored; select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.51.2 --24dzbv6kpqxe4yje-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-03-29 19:45 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-03-29 19:45 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 66622bf4837..1d98287af62 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8911,6 +8911,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -8974,29 +8975,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2567d918ec3..b3ee82f401e 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4943,6 +4943,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4986,6 +5213,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --tyerjrpxgsfvwown-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-04-24 08:44 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-04-24 08:44 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 75 +++++++ src/backend/commands/tablecmds.c | 46 +++-- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 232 +++++++++++++++++++++- src/test/regress/sql/alter_table.sql | 147 ++++++++++++++ 5 files changed, 481 insertions(+), 21 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..cfb0a0068cf 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -17,6 +17,7 @@ #include "access/genam.h" #include "access/gist.h" #include "access/htup_details.h" +#include "access/relation.h" #include "access/sysattr.h" #include "access/table.h" #include "catalog/catalog.h" @@ -29,6 +30,8 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" +#include "parser/parse_relation.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +697,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index aa54c629f8b..c886e6e5ca7 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8938,6 +8938,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, NewColumnValue *newval; RawColumnDefault *rawEnt; Relation pg_attribute; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9001,29 +9002,36 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, /* Make above changes visible */ CommandCounterIncrement(); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); - - /* Make above changes visible */ - CommandCounterIncrement(); - - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); - /* Build a concrete expression for the new default (generated) value */ defval = (Expr *) build_column_default(rel, attnum); defval = expression_planner(defval); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = defval; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + (Node *) defval); + + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); + + /* Make above changes visible */ + CommandCounterIncrement(); + + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = defval; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 08981f4e380..5346791fd8d 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4955,6 +4955,233 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; t (1 row) +drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; + a | b | expected | correct +---+---+----------+--------- + 1 | 2 | 2 | t + 2 | 4 | 4 | t +(2 rows) + +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; + a | b | expected | correct +---+----+----------+--------- + 3 | 6 | 6 | t + 4 | 8 | 8 | t + 5 | 10 | 10 | t +(3 rows) + +rollback; +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +ERROR: cannot change inherited column to be a stored generated column +rollback; +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +ERROR: ALTER TABLE / ADD GENERATED ALWAYS AS (expr) STORED must be applied to child tables too +rollback; +drop table testgen.tpart; -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; @@ -4998,6 +5225,7 @@ alter table testgen.t3 alter column b ERROR: cannot use subquery in column generation expression drop table testgen.t3; drop schema testgen cascade; -NOTICE: drop cascades to 4 other objects -DETAIL: drop cascades to table testgen.t3 +NOTICE: drop cascades to 3 other objects +DETAIL: drop cascades to table testgen.t5 +drop cascades to function testgen.gen() drop cascades to table testgen.t1 diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 76187083289..efb08b9ff70 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,153 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 + +-- test support for partitioned tables and inheritance +create table testgen.tpart (a int, b int) partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) select x, x from generate_series(1, 5) x; + +-- altering the parent table, recursing +begin; +alter table testgen.tpart alter column b + add generated always as (a * 2) stored; +-- expected: all the partitions have been rewritten +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p1 order by a; +select a, b, a * 2 as expected, b = (a * 2) as correct + from testgen.tpart_p2 order by a; +rollback; + +-- altering a single partition is not allowed +begin; +-- expected: error +alter table testgen.tpart_p1 alter column b + add generated always as (a * 2) stored; +rollback; + +-- altering only the parent table is not allowed +begin; +-- expected: error +alter table only testgen.tpart alter column b + add generated always as (a * 2) stored; +rollback; + +drop table testgen.tpart; + +-- subpartitions +create table testgen.tpart (a int, b int, c int) + partition by hash (a); +create table testgen.tpart_p1 partition of testgen.tpart + for values with (modulus 2, remainder 0) + partition by hash (b); +create table testgen.tpart_p1_1 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p1_2 partition of testgen.tpart_p1 + for values with (modulus 2, remainder 1); +create table testgen.tpart_p2 partition of testgen.tpart + for values with (modulus 2, remainder 1) + partition by hash (b); +create table testgen.tpart_p2_1 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 0); +create table testgen.tpart_p2_2 partition of testgen.tpart_p2 + for values with (modulus 2, remainder 1); +insert into testgen.tpart (a, b) + select x, y + from generate_series(1, 5) x + cross join generate_series(1, 5) y; +-- currently, it is not possible to change the generated state of an +-- inheritance tree of depth >= 2 (same as in DROP EXPRESSION), so we expect an +-- error here. This might be fixed later. +begin; +alter table testgen.tpart alter column c + add generated always as (a + b) stored; +rollback; + +drop table testgen.tpart; + -- tests for invalid invocations alter table doesnotexist alter column foo add generated always as (bar * 2) stored; -- 2.47.0 --4wx636ozsu2eakgd-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
* [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column @ 2026-05-14 21:51 Alberto Piai <[email protected]> 0 siblings, 0 replies; 425+ messages in thread From: Alberto Piai @ 2026-05-14 21:51 UTC (permalink / raw) This builds upon basic support for ... ALTER COLUMN ... ADD GENERATED ALWAYS AS (expr) STORED If we can find a constraint which proves that the given column is already always equal to the new generated column expression, skip the expensive rewrite of the table. The check constraint must use an equality operator which is mergejoinable, and the expression must match exactly the generated column's default expression. --- src/backend/catalog/pg_constraint.c | 73 +++++++++++ src/backend/commands/tablecmds.c | 40 +++--- src/include/catalog/pg_constraint.h | 2 + src/test/regress/expected/alter_table.out | 153 ++++++++++++++++++++++ src/test/regress/sql/alter_table.sql | 83 ++++++++++++ 5 files changed, 335 insertions(+), 16 deletions(-) diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c index b12765ae691..37628f65dc1 100644 --- a/src/backend/catalog/pg_constraint.c +++ b/src/backend/catalog/pg_constraint.c @@ -29,6 +29,7 @@ #include "catalog/pg_type.h" #include "commands/defrem.h" #include "common/int.h" +#include "nodes/nodeFuncs.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/fmgroids.h" @@ -694,6 +695,78 @@ findDomainNotNullConstraint(Oid typid) return retval; } +/* + * Given a relation, an attnum and a (cooked) expression, this returns true if + * it finds a CHECK constraint which proves that the given column is equal to + * the expression. + * + * The constraint must use a mergejoinable operator for the type of the column, + * a concept used by the planner as well to infer equivalence classes on the + * terms in a query (see op_mergejoinable()). + * + * The expressions are compared structurally, so they must match exactly for + * this check to succeed. + */ +bool +findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr) +{ + Relation pg_constraint; + HeapTuple conTup; + SysScanDesc scan; + ScanKeyData key; + bool found = false; + + pg_constraint = table_open(ConstraintRelationId, AccessShareLock); + ScanKeyInit(&key, + Anum_pg_constraint_conrelid, + BTEqualStrategyNumber, F_OIDEQ, + ObjectIdGetDatum(relid)); + scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, + true, NULL, 1, &key); + + while (HeapTupleIsValid(conTup = systable_getnext(scan))) + { + Form_pg_constraint con = GETSTRUCT(conTup); + char *conbin; + Datum val; + Node *conexpr; + + if (con->contype != CONSTRAINT_CHECK) + continue; + if (!con->convalidated) + continue; + + val = SysCacheGetAttrNotNull(CONSTROID, conTup, + Anum_pg_constraint_conbin); + conbin = TextDatumGetCString(val); + conexpr = stringToNode(conbin); + + if (IsA(conexpr, OpExpr)) + { + OpExpr *op = (OpExpr *) conexpr; + + if (list_length(op->args) == 2 && IsA(linitial(op->args), Var)) + { + Var *var = linitial(op->args); + + if (var->varattno == attnum && + op_mergejoinable(op->opno, exprType((Node *) var)) && + equal(lsecond(op->args), target_expr)) + { + found = true; + break; + } + } + } + } + + systable_endscan(scan); + table_close(pg_constraint, AccessShareLock); + + return found; +} + /* * Given a pg_constraint tuple for a not-null constraint, return the column * number it is for. diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index 39faef0a114..4afa157ee9e 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -8939,6 +8939,7 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, Relation pg_attribute; List *newcons; CookedConstraint *cookedDef; + bool rewrite; Assert(def->raw_expr != NULL); Assert(def->cooked_expr == NULL); @@ -9013,25 +9014,32 @@ ATExecAddGeneratedAsExprStored(AlteredTableInfo *tab, cookedDef = linitial(newcons); - /* - * Clear all the missing values if we're rewriting the table, since this - * renders them pointless. - */ - RelationClearMissing(rel); + rewrite = !findStructuralCheckConstraintOnAttr(RelationGetRelid(rel), + attnum, + cookedDef->expr); - /* Make above changes visible */ - CommandCounterIncrement(); + if (rewrite) + { + /* + * Clear all the missing values if we're rewriting the table, since + * this renders them pointless. + */ + RelationClearMissing(rel); - /* Drop any pg_statistic entry for the column */ - RemoveStatistics(RelationGetRelid(rel), attnum); + /* Make above changes visible */ + CommandCounterIncrement(); - /* Schedule a rewrite */ - newval = palloc0_object(NewColumnValue); - newval->attnum = attnum; - newval->expr = (Expr *) cookedDef->expr; - newval->is_generated = true; - tab->newvals = lappend(tab->newvals, newval); - tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + /* Drop any pg_statistic entry for the column */ + RemoveStatistics(RelationGetRelid(rel), attnum); + + /* Schedule a rewrite */ + newval = palloc0_object(NewColumnValue); + newval->attnum = attnum; + newval->expr = (Expr *) cookedDef->expr; + newval->is_generated = true; + tab->newvals = lappend(tab->newvals, newval); + tab->rewrite |= AT_REWRITE_DEFAULT_VAL; + } InvokeObjectPostAlterHook(RelationRelationId, RelationGetRelid(rel), attnum); diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h index 1b7fedf1750..7ac9e00c28b 100644 --- a/src/include/catalog/pg_constraint.h +++ b/src/include/catalog/pg_constraint.h @@ -266,6 +266,8 @@ extern char *ChooseConstraintName(const char *name1, const char *name2, extern HeapTuple findNotNullConstraintAttnum(Oid relid, AttrNumber attnum); extern HeapTuple findNotNullConstraint(Oid relid, const char *colname); extern HeapTuple findDomainNotNullConstraint(Oid typid); +extern bool findStructuralCheckConstraintOnAttr(Oid relid, AttrNumber attnum, + const Node *target_expr); extern AttrNumber extractNotNullColumn(HeapTuple constrTup); extern bool AdjustNotNullInheritance(Oid relid, AttrNumber attnum, const char *new_conname, bool is_local, bool is_no_inherit, bool is_notvalid); diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out index 2d0ce414d12..b269736ab1a 100644 --- a/src/test/regress/expected/alter_table.out +++ b/src/test/regress/expected/alter_table.out @@ -4956,6 +4956,159 @@ select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; (1 row) drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) NOT VALID +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; + did_rewrite +------------- + t +(1 row) + +\d+ testgen.t4 + Table "testgen.t4" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | not null | generated always as (a * 3) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b >= (a * 2)) +Not-null constraints: + "t4_b_not_null" NOT NULL "b" + +drop table testgen.t4; +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessExclusiveLock +(1 row) + +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+-------------------------- + relation | ShareUpdateExclusiveLock +(1 row) + +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; + locktype | mode +----------+--------------------- + relation | AccessShareLock + relation | AccessExclusiveLock +(2 rows) + +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; + did_skip_rewrite +------------------ + t +(1 row) + +\d+ testgen.t5 + Table "testgen.t5" + Column | Type | Collation | Nullable | Default | Storage | Stats target | Description +--------+---------+-----------+----------+------------------------------------+---------+--------------+------------- + a | integer | | | | plain | | + b | integer | | | generated always as (a * 2) stored | plain | | +Check constraints: + "chk_gen_clause" CHECK (b = (a * 2)) + +drop function testgen.gen; +drop table testgen.t5; -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql index 51d818d4995..9851093fb58 100644 --- a/src/test/regress/sql/alter_table.sql +++ b/src/test/regress/sql/alter_table.sql @@ -3198,6 +3198,89 @@ select pg_relation_filenode('testgen.idx_b') as idx_filenode_after \gset select :idx_filenode_before != :idx_filenode_after as did_rewrite_idx; drop table testgen.t3; +-- turning a regular column into a stored generated column +-- without rewriting the table (when a check constraint proves it isn't needed) +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before = :t4_filenode_after as did_skip_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint is not +-- valid +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b = a * 2) not valid; +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 2) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- turning a regular column into a stored generated column +-- same as the previous case, but a rewrite happens since the constraint +-- operator is not mergejoinable +create table testgen.t4 (a int, b int not null); +insert into testgen.t4 (a, b) select x, x * 2 from generate_series(0, 5) x; +alter table testgen.t4 add constraint chk_gen_clause check (b >= a * 2); +select pg_relation_filenode('testgen.t4') as t4_filenode_before \gset +alter table testgen.t4 alter column b add generated always as (a * 3) stored; +select pg_relation_filenode('testgen.t4') as t4_filenode_after \gset +select :t4_filenode_before != :t4_filenode_after as did_rewrite; +\d+ testgen.t4 +drop table testgen.t4; + +-- test the whole process for adding a stored generated column without +-- long-lived exclusive locks +create table testgen.t5 (a int); +select pg_relation_filenode('testgen.t5') as t5_filenode_before \gset +insert into testgen.t5 select x from generate_series(1, 5) x; +alter table testgen.t5 add column b int; +-- take care of new and updated columns +create function testgen.gen () returns trigger language plpgsql as $$ +begin + new.b = new.a * 2; return new; +end +$$; +create trigger testgen_gen + before insert or update on testgen.t5 + for each row execute function testgen.gen(); +-- add the constraint as not valid: enforced only for new and updated rows +begin; +alter table testgen.t5 + add constraint chk_gen_clause check (b = a * 2) not valid; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +insert into testgen.t5 (a) values (100), (200), (300); +-- backfill existing rows at the appropriate pace +update testgen.t5 set b = a * 2 where b is null; +-- validate: this scans the table, but without an exclusive lock +begin; +alter table testgen.t5 validate constraint chk_gen_clause; +select locktype, mode from pg_locks + where relation = 'testgen.t5'::regclass and granted; +commit; +-- now the schema update, which skips the rewrite because of the check +begin; +drop trigger testgen_gen on testgen.t5; +alter table testgen.t5 alter column b + add generated always as (a * 2) stored; +select locktype, mode from pg_locks +where relation = 'testgen.t5'::regclass and granted; +commit; +select pg_relation_filenode('testgen.t5') as t5_filenode_after \gset +select :t5_filenode_before = :t5_filenode_after as did_skip_rewrite; +\d+ testgen.t5 +drop function testgen.gen; +drop table testgen.t5; + -- test support for partitioned tables and inheritance create table testgen.tpart (a int, b int) partition by hash (a); create table testgen.tpart_p1 partition of testgen.tpart -- 2.47.0 --zfrmj4necy5zy3mo-- ^ permalink raw reply [nested|flat] 425+ messages in thread
end of thread, other threads:[~2026-05-14 21:51 UTC | newest] Thread overview: 425+ messages (download: mbox mbox.gz follow: Atom feed) -- links below jump to the message on this page -- 2024-03-29 15:43 [PATCH v9] Avoid orphaned objects dependencies Bertrand Drouvot <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-16 23:28 [PATCH v1 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-03-29 19:45 [PATCH v2 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-04-24 08:44 [PATCH v3 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]> 2026-05-14 21:51 [PATCH v4 2/2] Try to avoid a rewrite when adding a stored generated column Alberto Piai <[email protected]>
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